Maintenance of genomic integrity is essential to prevent the accumulation of mutations and genomic rearrangements that predispose to developmental disorders and cancer. Double-strand breaks (DSBs) are potentially lethal DNA lesions induced by DNA damaging agents, such as ionizing radiation, or spontaneously formed during DNA replication and transcription. Following DSB formation, activation of the DNA damage response (DDR) promotes genomic integrity. The ATM kinase is a principal DDR component that maintains genomic stability in response to DSBs by regulating a large number of cellular activities, ranging from DNA repair and replication to splicing and transcription. ATM cooperates with the Nijmegen breakage syndrome protein NBS1 to suppress ribosomal DNA (rDNA) transcription after ionizing radiation. We have recently shown that in response to ionizing radiation NBS1 localizes to nucleoli, where it associates with the Treacher Collins syndrome protein TCOF1 to suppress rDNA transcription. Despite these important observations, a complete understanding of the mechanisms by which rDNA transcription is regulated after ionizing radiation is currently lacking. The goal of this proposal is to elucidate how ATM cooperates with NBS1 and TCOF1 to suppress rDNA transcription after ionizing radiation and examine how defective inhibition of rDNA transcription causes genomic instability. In particular, we propose: 1) to elucidate the mechanisms by which ATM and CK2 regulate the interaction between NBS1 and TCOF1 after ionizing radiation; 2) to determine the processes through which ATM, NBS1 and TCOF1 suppress ribosomal DNA transcription in response to ionizing radiation; 3) to define the mechanisms by which the ATM-NBS1-TCOF1 pathway maintains genomic stability. Our approach will combine molecular and cell biology techniques with proteomic and biochemical assays to provide a detailed understanding of the molecular events that lead to and result from the inhibition of rDNA transcription mediated by ATM, NBS1 and TCOF1 after ionizing radiation. We anticipate that our studies will elucidate novel processes through which ATM and NBS1 ensure the stability of the genome in response to ionizing radiation.